1. Field of the Invention
The present invention relates to a radiation curable resin composition which is rapidly cured by irradiating ultraviolet light, and, more particularly, to a radiation curable resin composition applicable in the manufacture of optical parts such as lenses, optical disks, prisms, and lens sheets which are molded by polymerization in dies. The resin composition is also suitable for use as a surface coating material of plastic film substrates used for optical materials such as substrates for a liquid crystal display element.
2. Description of Related Art
A method for manufacturing plastic lenses and the like by irradiating a UV curable resin with ultraviolet light from a mercury lamp as an activated energy ray has been proposed (Japanese Patent Applications Laid-open No. 194401/1986, No. 207632/1988). According to this method, although there is an advantage that the resin is cured in a short period of time by irradiation of ultraviolet light, transparency and the color of the lens produced by curing with irradiation of ultraviolet light are generally inferior in comparison with those produced from a polymer such as polymethyl methacrylate, polycarbonate, or diethylene glycol bisarylcarbonate. In addition, coloration of the molded products upon exposure to ultraviolet light is a problem, moreover, in the molding by polymerization in dies of conventional UV curable resins, a distortion at the time of curing tends to remain in the molded product after molding, so that the molded product may warp or shrink after the product has been removed from the die, impairing the dimensional accuracy of the product.
Japanese Patent Application Laid-open No. 174910/1995 proposes a prism sheet which increases front brightness of a back light unit which is used in liquid crystal display equipment. This prism sheet is molded in a specific shape from a transparent material with of a specific refractive index. As the methods for producing the prism sheet, the patent discloses a method for integrated molding using a transparent glass possessing a refractive index of a specific range and a method for molding a prismatic shape from a UV curable resin composition.
As admitted by the Japanese Patent Application Laid-open No. 174910/1995, plastic materials represented by these UV curable resins can be used only limitedly to the applications requiring heat resistance. In other words, if a lens sheet typified by the prism sheet obtained from a conventional UV curable resin is used in a high temperature environment, a part of the lens sheet fuses on the surface of the back light and leaves a sticking scar. This is a fatal drawback affecting the optical performance of the products.
In addition, from the viewpoint of making thinner and larger liquid crystal cells having a curved display surface, attention is given to the liquid crystal display element using a plastic film as a substrate instead of the conventional glass substrate. However, when a plastic film is used as a substrate, the external surface of the substrate is easily damaged, impairing the external appearance and inviting occurrence of irregular reflection due to the wound. The optical characteristics of the liquid crystal display element is thus adversely affected. For this reason, a coating layer of a UV curable resin is provided onto the surface of the substrate to increase the hardness of the substrate surface, thereby obtaining a liquid crystal display element of which the surface is damaged only with difficulty.
However, if said coating layer is formed using a conventional UV curable resin, the resulting image is dark and poor as compared with those obtained from the liquid crystal display element using a glass substrate due to insufficient transparency, coloration resulting from exposure to light, and the like.
The present invention has been completed in view of these drawbacks in conventional technology and has an object of providing a radiation curable resin composition suitable for molding, which exhibits a high productivity and high transparency, and can produce molded products with a high refractive index, high surface hardness, minimal coloration due to exposure to lights, excellent dimensional precision, superior heat resistance, and freedom from adhesion to the surface in a high temperature environment.